For many years seismic exploration for oil and gas has involved the use of a source of seismic energy and its reception by an array of seismic detectors, generally referred to as geophones on land, and hydrophones at sea. In land surveys, the source of seismic energy can be a high explosive charge electrically detonated in a borehole located at a selected point on the terrain, or a vibrator truck that creates a series of vibrations or impacts of a baseplate to the earth's surface. At sea, the most common source is an air gun.
The acoustic waves generated in the earth by these various seismic sources are partially reflected from various earth layers and transmitted back from layer boundaries and reach the surface of the earth at varying intervals of time, depending on the distance and characteristics of the subsurface traversed. These returning waves are detected by the receivers, which function to transduce such acoustic waves into representative electrical signals.
Conventionally, an arrangement of receivers is generally laid out along a line to form a series of observation stations within a desired locality. The seismic source applies an acoustic signal to the earth, and the detected signals, which are reflected from points midway between the source and receiver, are recorded for later processing.
These recorded signals or “traces” are continuous electrical analog signals depicting amplitude versus time, and are generally quantized using digital computers so that each data sample point may be operated on individually.
The receiver arrangement is then moved along the line to a new position where some of the shot or receiver points may overlap, and the process repeated. With enough repeats, a seismic survey is obtained. If the ground and subsurface reflecting layer are flat, as previously mentioned a seismic shot yields data from midway between the source and receiver.
One of the techniques utilized in processing seismic data is to combine traces produced from two or more shots wherein the midpoint between the source and the receiver in each case is the same, although the offset, i.e., sources to receiver distance, may be different. When two or more traces belonging to a common midpoint (CMP) are summed, the technique is called common-midpoint stacking.
A single wave-producing activation of a source (regardless of the source type), called a “shot,” results in generating a number of traces equal to the number of receivers. Aligning all of the recorded traces from a single shot in a side by side display i.e. a “shot gather” can produce a rudimentary two dimensional seismic section. The section can be improved, however, by the CMP stacking. Since sound traveling two different paths gives information from approximately the same subsurface point, two such traces reflected from a common point can be combined, i.e., summed, such that reflection amplitudes are added but the noise, which occurs at different times on the two traces, is not added thus improving the signal-to-noise ratio. The number of traces summed in an individual stack is called the multifold or simply the “fold.”
More recently, seismic surveys involve receivers and sources laid out in more complex geometries, generally involving rectangular or non-orthogonal grids covering an area of interest so as to expand areal coverage and enable construction of three-dimensional (3D) views of reflector positions over wide areas.
A normal prior art three-dimensional survey geometry is shown in FIG. 1, in which a basic grid, indicated generally at 21, is defined for effective placement of shotpoints that are designated as squares 24, and geophone receivers that are designated as crosses 22. As illustrated, the basic grid 21 is a square having a dimension d1 that is equal to twice the desired reflection midpoint spacing, and that will provide an image having a desired resolution of subsurface features.
A plurality of geophone receiver lines 20a-20n each containing a plurality of equally spaced apart geophone receivers 22 is place in parallel on the earth's surface. A plurality of shotpoints 24 is placed along source or shot lines 26a-26n which run orthogonally to the receiver lines 20a-20n, thus providing a symmetrical crossed array geometry with geophone receivers 22 in lines 20a-20n and source stations 24 in lines 26a-26n spaced apart a distance equal to d1, and the lines 26a-26n spaced apart a distance of four times d1 (4d1). This crossed-arrayed geometry produces subsurface spatial resolution in which midpoints are spaced apart by one-half of the distance d1 in the receiver line, and one-half of the distance d1 in the source line. For example, if receivers and sources, as shown in FIG. 1, are each spaced 165 ft. apart, reflection midpoints will be spaced apart by 82.5 ft. and four adjacent midpoints will form a square.
It is well known, however, to those skilled in the art that improved surface sampling resolution in a survey can be obtained with a source/receiver geometry that is referred to herein as “true 3D coverage.” This geometry also uses CMP stacking in which the shotpoints and receivers are laid out in the generally rectangular areas similar to the arrangement shown in FIG. 1, but with closer spacing of the receiver lines 20 in the shotpoint line direction. As used herein, a true 3D seismic source/receiver geometry locates a geophone receiver and/or a shotpoint at each intersection of the basic grid 21.
An example of true 3D seismic source/receiver geometry having a geophone receiver at each intersection of the basic grid 21, and having shotpoints spaced apart at a distance four times d1 is illustrated in FIGS. 2A and 2B. Using the same size basic grid 21 as shown in FIG. 1, such a true 3D layout would include 400 geophone receivers and 25 shotpoints covering a surface area 19 in FIG. 2A that is equal to the surface area 19 shown in FIG. 1. Once all of the receivers and shotpoints are in place, the shots are sequentially activated and a number of traces that is equal to the number of shots times the number of receivers are recorded to provide a single data set from which a display of a seismic 2D section, or a 3D volume could be produced. In this true 3D technique the recorded traces having common midpoints, which are sorted out later from the recorded traces, are gathered in a display, which yield greatly increased surface resolution compared to the surface resolution shown in FIG. 1.
In seismic acquisition and processing operations, it is well known to those skilled in the art that a frequency ambiguity called aliasing is inherent in sampling systems, and that aliasing occurs in a sampling process when there are fewer than two samples per cycle. Aliasing applies to both the time and space domains. The aliasing that is done by the separated elements of geophone receivers and shotpoints is called spatial aliasing and depends on the surface spacing of the shotpoints and receiver. The aliasing that is done by sampling an input signal is called frequency aliasing and is dependent on the sampling interval used to digitize input signals.
To avoid aliasing, filtering is commonly required. For example, an alias filter applied before sampling a geophone signal at a ground location removes certain undesired frequencies, likewise a velocity filter of a seismic gather attenuates certain coherent arrivals of waves, which sweep over the geophone receivers having certain apparent receiver velocities. Accordingly, an advantage of true 3D seismic source/receiver layout geometry is avoiding spatial aliasing.
There is a disadvantage to this kind of true 3D shooting, however, in the excessive amount of equipment required to occupy every surface location with a receiver and/or a source on a grid interval equal to twice the desired subsurface resolution. Today, however, a normal 3D seismic survey based on a layout geometry similar to FIG. 1 is an accepted part of the early data-acquisition process because the high resolution display of 3D surveys leads to an optimized appraisal of sites, refined reserve estimates, and more efficient development plans. Accordingly, the benefits of a 3D survey using source receiver geometry similar to that shown in FIG. 1, although having certain known deficiencies, usually outweigh the additional cost compared to a 2D survey. Accordingly, if use of 3D seismic surveys is to continue to grow, a need exists for new and improved methods that simplify and/or provide economical alternatives that reduce the operational cost of obtaining a 3D seismic survey.
U.S. Pat. No. 6,026,058 by Phillips Petroleum attempted to solve some of these issues by applying what is called a “hybrid gather” method. In this method, full survey data from a crossed-array source/receiver layout geometry is assembled from a series of hybrid gathers that are centered at the intersections of the crossed-array source/receiver lines.
A hybrid gather, as described in U.S. Pat. No. 6,026,058, is a side-by-side grid display of seismic traces corresponding to a gather center located within in a larger 3D crossed-array seismic grid layout and includes traces selected from sectional parts of the larger layout. Accordingly the hybrid gather is a singlefold CMP gather center about a source/receiver line intersection, which has a desired spatial resolution in both shot line and receiver line directions.
Steps in obtaining the hybrid gather include laying out the larger than normal crossline spacing 3-D crossed array source/receiver survey area, with a selected dimension d applied to the spacing between both the source lines and receiver lines and a smaller dimension d1 applied between sources and receivers in the respective source or receiver line. The dimensions d and d1 thus define the number of sources in a selected source line, and the number of receivers in a selected receiver line to be included in the hybrid gather corresponding to each hybrid gather center.
Next, a normal recording is made where a seismic source is energized at each shotpoint in the 3-D survey to induce seismic pulses into the earth, and the reflected seismic pulses generated by each of the geophone receivers are recorded for later processing.
A series of hybrid gathers is then obtained by performing hybrid gathers at each intersection of the crossed array of source and receiver lines of the larger 3-D survey area, such that when the series of hybrid gathers is complete, traces from the entire survey area can be gathered by assembling the series of hybrid gathers. Thus the spatial resolution associated with the fully assembled gather is equal to the spatial resolution of a true source/receiver layout.
However, this method did not work well with a limited number of receivers. Conventional approaches as defined by U.S. Pat. No. 6,026,058 are fundamentally geared around land seismic data, where commonly there are more receivers than shots. Hybrid gathers are best when the station spacing between the shots and the receivers are the same while the line spacing is not critical as each hybrid gather is independent. On a conventional land survey the station spacings are commonly the same and normally relatively small distances. In the marine case though, placing marine receivers is quite expensive and each unit is also expensive so they are normally coarsely laid out. The shots on the other hand tend to be very cheap so they are quite densely laid out. These conditions result in poor hybrid gather formation by the conventional approach.
Thus, what is needed in the art are better methods of collecting and processing seismic datasets, so as to further reduce noise, improve efficiencies and reduce costs.